The endothelins are a family of 21-amino acid peptides, e.g., ET-1, ET-2, and ET-3, originally characterized by their potent vasoconstricting and angiogenic properties (see, e.g., Luscher et al., 1995, Agents Actions Suppl. (Switzerland) 45: 237-253; Yanagisawa et al., 1988, Nature 332: 411-415). These peptides additionally appear to be related to growth factors such as bFGF and often act in synergy with them (see, e.g., Halaban, 1996, Seminars in Oncology 23: 673-681; Reid et al., 1996, Development 122: 3911-3919; Markewitz et al., 1995, Am. J. Physiol. 268: L192-L200; and Nelson et al., 1996, Cancer Res. 56: 663-668). Furthermore, these peptides display cytokine-like regulatory properties and can be influenced by hormones such as insulin and angiotensin II as well as growth factors such as TGF-.beta. and TNF-.alpha. (Nelson et al., supra; Suzuki et al., 1989, J. Biochem. 106: 736-741; and Lundblad et al., 1996, Crit. Care Med. 24: 820-826; ). Endothelin activity is mediated via binding with preferential affinities to two distinct G-coupled receptors, ETA and ETB, in an autocrine/paracrine manner (see, e.g., Hocher et al., 1997, Eur. J. Clin. Chem. Clin. Biochem. 35(3): 175-189; Shichiri et al., 1991, J. Cardiovascular Pharmacol. 17: S76-S78).
There are a variety of agonists and antagonists of endothelin receptors (Webb et al., supra), which have been used to study the mechanism of action of the endothelins. Because endothelin is known to have powerful vasoconstrictive activity, endothelin antagonists in particular (also termed "endothelin receptor antagonists" in the art) have been studied with regard to their possible role in treating human disease, most notably, cardiovascular diseases such as hypertension, congestive heart failure, atherosclerosis, restenosis, and myocardial infarction (Mateo et al., 1997, Pharmacological Res. 36 (5): 339-351). For example, non-peptide-based endothelin antagonists belonging to the pyrimidinyl sulfonamide family, such as Ro 46-2005 and bosentan, which interact with the endothelin receptor through their aromatic rings, are currently undergoing clinical evaluation for the treatment of hypertension, vascular disease, and congestive heart failure. These antagonists can bind both ETA and ETB with varying affinities and have advantages over peptide-based antagonists because they possess an improved metabolic stability (Webb et al., supra; and Parris et al., supra). In addition, endothelin antagonists have also been studied with regard to their possible role in the treatment of kidney disease such as impaired renal function in liver cirrhosis and acute renal failure (Gomez-Garre et al., 1996, Kidney Int. 50: 962-972; Hocher et al., supra).
More recently, endothelins and endothelin receptors have been implicated in a number of normal and pathological cell growth processes, e.g., cell cycle progression, cell growth, and cellular development (see, e.g., Parris et al., 1997, Vascular Medicine 2: 31-43; Markewitz et al., supra; Morbidelli et al., 1995, Am. J. Physiol. 269: H686-H695; and Battistini et al. 1993, Peptides 14: 385-399). ET1 and ET3 have been shown to be mitogenic and chemokinetic factors for normal tissues ranging from endothelial and epithelial cells to macrophages (see, e.g., Webb et al., 1997, Medicinal Research Reviews 17 (1): 17-67; and Gomez-Garre et al., supra). In addition, the binding of endothelins to their receptors has been shown to cause DNA synthesis, proliferation and cell mobilization in normal and neoplastic cells (Webb et al., supra; Ziche et al., 1995, Cardiovasc. Pharmacol. 26: S284-S286; and Yamashita et al., 1991, Res. Comm. in Chem. Pathol. and Pharmacol. 74 (3): 363-369).
This potential capability of endothelins to mediate cell growth and cell cycle progression has led to some initial studies of endothelin expression and/or endothelin receptor presence in cancer cells. For example, ET-1 has been shown to be overexpressed in breast cancer and pancreatic cell lines and induces proliferation in breast cancer tissue, ovarian cell lines and prostate tumors (see, e.g., Moriatis et al., 1997, Eur. J. Canc. 33 (4): 661-668; Nelson et al., 1996, Cancer Res. 56: 663-668; Patel et al., 1995, Br. J. Cancer 71: 442-447; Oikawa et al., 1994, Br. J. Cancer 69: 1059-1064; Shichiri et al., supra; and Yamashita et al., supra). In addition, the presence of ETA type receptors, which have a higher affinity for ET1 and ET2, has been demonstrated in ovarian cell lines (Moriatis et al., supra) and breast cancer tissues (Yamashita et al., supra). One of the few tumors to express ETB receptors that have a similar affinity for all three isoforms of endothelin is melanoma (Yohn et al., 1994, Biochem. Biophys. Res. Comm. 201 (1): 449-457). Interestingly, ETB receptors are highly expressed in primary or recurrent melanomas but less so in metastatic melanomas (Kikuchi et al., 1996, Biochem. Biophys. Res. Comm. 219: 734-739).
Although these studies suggest that endothelin antagonists could potentially have therapeutic applications in the treatment of cancer, there have been no studies to date demonstrating any such therapeutic application. In fact, the role that endothelin may play in promoting proliferative disease such as various vascular proliferative diseases and benign prostatic hypertrophy (BPH) is unclear (Webb et al., supra and Kenny et al., 1997, J. Med. Chem. 40 (9): 1293-1315). Moreover, while U.S. Pat. Nos. 5,550,110 and 5,641,752 disclose the use of specific hexapeptide endothelin antagonists for the treatment of cancer, there is actually no data in those disclosures relating to cancer treatment and no indication as to how to perform such treatment or indeed whether such treatment would be successful (see also, PCT applications WO 97/37987, 97/08169, WO 96/11927, and WO 94/03483, Canadian patent application 2072395, and U.S. Pat. No. 5,658,943).